A well-known problem in ship propulsion and particularly waterjet propulsion systems, is maintaining efficiencies of a water vehicle and the propulsion system used under all speed and power requirements. One scheme used to maintain efficiency is a plurality of waterjet propulsion units in parallel. Such a parallel arrangement is desirable where a vehicle's power requirements exceed the power available from one unit. In parallel arrangements, each unit normally has its own separate intake, pump, engine, and waterjet. Another variation is to use only one inlet with downstream ducting connected to provide fluid to parallel and separately powered pumps with their separate nozzles. Partial power operation is usually accomplished by changing intake size or by reducing rotational speed and power of the pumps below full-load, design valves, obviously degrading pump and flow efficiencies. Also, another scheme used to reduce power is to shut down one or more propulsion units, and operate the remaining one or more units at full design load, values with efficiency, provided asymetric thrust does not result. But again, efficiency of the vehicle is degraded, because of the drag of the unused intake area remaining open, and the mismatch of the total in-use nozzle area to vehicle hull requirements. When operating all the propulsion units at reduced power, as before, unit-power efficiency is usually drastically reduced, particularly in gas turbine-powered units such as those often used to power many high performance marine vehicles.
An alternative method of providing variable power is to couple two or more power units to each pump-nozzle-intake system, by power-combining transmission systems. Reduced power operation is obtained by power unit shaft speed control, or by decoupling one or more power units. The latter method requires shaft clutches and controls. And, as with individual propulsion units discussed above, power unit efficiency is degraded at reduced power operation, due to shaft speed-to-power mismatch of the pump and power-unit and the losses due to the unused thrust nozzle area. The major disadvantages of the system are the cost, the complexity of the power-combining transmissions, and the restraints placed on their location and arrangement in the vehicle.